Chromaffin cells of the adrenal medulla exhibit remarkable phenotypic plasticity. These cells normally exhibit an endocrine phenotype, synthesizing and secreting epinephrine and norepinephrine. When they are removed from the influence of the adrenal cortex they differentiate to a neuronal phenotype extending processes and forming synapses. This transdifferentiation has been demonstrated upon transplant to the brain, anterior chamber of the eye, kidney capsule and in primary tissue culture. In each case the cells extend processes and interact with each other and, in the case transplants, with the resident cell population. Transplantation of these cells to the substantia nigra has been shown to be efficacious for alleviating Parkinson's disease-like symptoms in a commonly used rat model for this degenerative disease. A thorough understanding of the differentiation of these cells is of potential clinical importance in addition to providing an excellent model for the study of gene regulation during differentiation in general. The long term goal of the studies described here is to identify and characterize the cis-acting elements that regulate the expression of specific structural genes during differentiation of adrenal chromaffin cells. We will first define the repertoire of proteins whose expression changes during this differentiation by 2D gel analysis. We will then isolate and characterize cDNA clones coding for several of these proteins. One or two genes will then be isolated whose expression is characteristic of the endocrine and neuronal phenotypes and their regulation will be studied by gene transfer into an appropriate recipient cell line. The pheochromocytoma cell line, PC12, is an excellent candidate for such a recipient. These cells normally exhibit an endocrine phenotype very similar to that of the chromaffin cells from which they arose. When they are treated with nerve growth factor they stop growing and extend processes very much like normal chromaffin cells. This system, therefore, provides an excellent model in which in vitro mutagenesis and gene transfer techniques can be used to identify sequences that are important for the regulation of specific genes during development.